While the hereinafter described invention was conceived for use in reducing the noise in the cabin of an aircraft, it is to be understood that the invention can be used to reduce noise and vibration in other types of structures. This includes all types of transportation vehicles--automobiles, buses, trucks, ships, submarines, hovercraft and hydrofoils, for examples. The invention can also be used in the exterior and interior walls of buildings and other enclosures where noise reduction is desired. Moreover, the skin whose vibration is to be damped may be the exterior skin of a structure or it may be the skin of interior items, such as interior walls, ceilings or floors (frequently referred to as the deck in transportation vehicles).
It is also to be understood that, because interior noise is reduced by damping skin vibrations, coincidental to the reduction of noise is a corresponding improvement in the sonic fatigue life of the skin and items attached to the skin. That is, reducing vibration not only reduces noise, it also improves the sonic fatigue life of the vibrating structure and items attached thereto.
In the past, the most common way of reducing skin vibration in commercial aircraft has been to apply skin damping tape to selected skin surfaces. Skin damping tape comprises a layer of viscoelastic material and a thin constraining layer, formed of aluminum foil, for example. The layer of viscoelastic material has adhesive properties, whereby the damping tape can be directly attached to the selected surface of the skin. In some instances several layers of damping tape are applied on top of one another to the region of the skin whose vibrations are to be damped. As the skin vibrates, shear strain is induced in the viscoelastic material, which causes the viscoelastic material to dissipate energy.
The use of damping tape of the type briefly discussed above has two major disadvantages. First, the use of damping tape adds an undesirably large amount of weight to the resulting structure for the amount of damping produced. This is particularly true when several layers of damping tape are required to obtain an adequate amount of noise reduction. Second, the application of damping tape is more labor intensive than desirable for the amount of damping produced. Again, this is particularly true when several layers of damping tape are required to obtain an adequate amount of noise reduction.
Another way of reducing skin vibration in commercial aircraft that has been used in the past is the addition of stiffeners. In some aircraft, stiffeners, plus one or more layers of damping tape have been used. For example, in the past the soundproofing in the upper deck of a Model 747 aircraft produced by The Boeing Company, Seattle, Wash., has included several hundred stiffeners and two layers of damping tape installed in selected areas. As with the use of skin damping tape alone, the use of stiffeners with or without damping tape adds an undesirably large amount of weight to the aircraft for the resultant amount of damping, and is more labor intensive than desirable.
Regardless of the approach used, the addition of large amounts of weight to achieve a desired objective, i.e., noise reduction, is disadvantageous in a transportation environment because any additional weight either increases fuel consumption, decreases payload over the lifetime of the transportation vehicle, or effects both parameters. While fuel consumption is, of course, of considerable importance in all vehicular environments, it is most critical in aircraft environments due to the already high fuel/payload ratio. And, of course, in any production environment, the use of high labor intensive approaches to solve any problem undesirably increases the price of the resultant product.
Various proposals have been made to reduce vibration and, thus, noise in aircraft and other environments. One such proposal is described in the U.S. patent application Ser. No. 79,325 filed Sept. 27, 1979 and entitled "Method and Apparatus for Wideband Vibration Damping of Reinforced Skin Structures" by Lloyd D. Jacobs, et al. (now abandoned) and U.S. patent application Ser. No. 297,945 filed Aug. 31, 1981 and entitled "Method and Apparatus for Wideband Vibration Damping of Reinforced Skin Structures" by Lloyd D. Jacobs et al., which is a confirmation-in-part of U.S. application Ser. No. 79,325. In this proposal a stiff brace is attached with a viscoelastic adhesive at one end to the skin of a reinforced skin structure and at the other end to a structural element of the reinforced skin structure. Vibration of the skin is directly damped by the layer of viscoelastic material attaching the brace to the reinforcing member. The indirect vibration damping assistance to the direct vibration damping allows the amount of direct vibration damping to be decreased. Another proposal is described in U.S. patent application Ser. No. 306,951, filed Sept. 30, 1981 and entitled "Viscoelastically Damped Reinforced Skin Structures" by Lloyd Jacobs. This proposal also uses direct and indirect damping, except that the coupling element is a layer of honeycomb or foam material rather than a brace and an overlying panel. The honeycomb or foam material provides thermal insulation as well as assists in vibration damping and noise reduction.
While the foregoing proposals have certain advantages in some environments, their weight and cost is such that they are unsatisfactory in other environments.